target/arm: Implement fp16 for Neon float-integer VCVT

target/arm: Implement fp16 for Neon float-integer VCVT

Convert the Neon float-integer VCVT insns to gvec, and use this
to implement fp16 support for them.

Note that unlike the VFP int<->fp16 VCVT insns we converted
earlier and which convert to/from a 32-bit integer, these
Neon insns convert to/from 16-bit integers. So we can use
the existing vfp conversion helpers for the f32<->u32/i32
case but need to provide our own for f16<->u16/i16.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200828183354.27913-37-peter.maydell@linaro.org